This proposal is for studies on aspects of cortical development which will focus on the roles played by eliminatory phenomena in the shaping of the mature cortex and on the emergence of distinctive projection patterns from various cortical neuronal populations and from various cortical regions. The proposed studies on eliminatory phenomena in cortical development center on the transient occipital cortical component of the corticospinal tract which my colleagues and I have previously shown is present during the early postnatal development in the rat, but is subsequently completely removed through collateral elimination. I am now proposing to investigate what factors may be involved in the normal removal of these transient occipital corticospinal collaterals by determining if they can be maintained by experimental manipulations in the neonate which will: (a) remove the definitive corticospinal neurons; (b) remove the sites to which the occipital cortical neurons with transient corticospinal axons normally maintain a collateral; (c) induce somatosensory input into the lateral geniculate and thus into the occipital cortex; (d) bring about a combination of these effects; or (e) provide, by transplantation, the transient occipital corticospinal axons with a target tissue to which their cells of origin normally maintain a projection. I also propose to extend my study of cortical cell death into the neocortex, since our recent results indicate that in the archicortex, cell death plays little, if any, role. I also propose to further study two phenomena we have recently identified: (1) the massive innervation of the lateral geniculate by medial lemniscal fiber in congenitally blind mice following neonatal somatosensory cortical lesions; and (2) a large yet transient projection from the subicular complex into the midbrain tegmentum. Further, I propose to use the retrogradely transported fluorescent dyes to study the emergence of the separate populations of projection neurons during the development of the occipital and parietal cortex. Finally, I propose using heterotopic cortical transplants to determine which, if any, of the projections characteristic of neurons in one region of cortex can be made by those neurons when they are put into a different region and which of the projections characteristic of the host region these neurons will extend.